Development electrode control apparatus

ABSTRACT

Apparatus is herein disclosed for controlling the quality of image development produced by a xerographic developing apparatus. Circuit means are provided to periodically sample a reference voltage on an exposed xerographic plate prior to image development and to provide an output signal indicative of the sampled voltage. The output signal is then applied to an adjustable power supply which is operatively connected to the development electrode and the electrode potential regulated in response to the reference voltage.

United States Patent Inventors Samuel Coriale Webster, N.Y.; Ned J. Seachman, Penfield, NJ. Appl. No. 854,086 Filed Aug. 29, 1969 Patented Oct. 12, 1971 Assignee Xerox Corporation Rochester, N.Y.

DEVELOPMENT ELECTRODE CONTROL APPARATUS 7 Claims, 5 Drawing Figs.

US. Cl 118/4, 117/175, 118/637 Int. Cl 603g 13/08 Field ofSearch 1 18/4, 636, 637; l17/17.5

References Cited UNITED STATES PATENTS 7 9x487 10/1960 Giaimo, Jr. 118/637 UX 3,147,147 9/1964 Carlson 1 18/637 3,412,710 11/1968 Robinson. 118/637 3,416,494 12/1968 Hudson 1 18/637 3,424,131 1/1969 Aser et a1 117/17.5 X 3,430,606 3/1969 Pease et a1 1 18/637 3,506,259 4/1970 Caldwell et al. 117/17.5 X

Primary Examiner-Mervin Stein AttorneysPau1 M. Enlow, Norman E. Schrader, James .1.

Ralabate, Ronald Zibelli and Thomas .1. Wall ABSTRACT: Apparatus is herein disclosed for controlling the quality of image development produced by a xerographic developing apparatus. Circuit means are provided to periodically sample a reference voltage on an exposed xerographic plate prior to image development and to provide an output signal indicative of the sampled voltage. The output signal is then applied to an adjustable power supply which is operatively connected to the development electrode and the electrode potential regulated in response to the reference voltage.

PATENT EU 012T] 2197:

SHEET 1 OF 3 INVENTORS SAMUEL D. CORIALE NED J. SEACHMAN ATTORNEY PATENIED 01:11 2M SHEU 2 OF 3 mm mm I bow vm W l. km um mm X Q PATENTEUIJEI12 I971 SHEET 3 OF 3 v HQK DEVELOPMENT ELECTRODE CONTROL APPARATUS This invention relates, in general, to xerographic developing apparatus and, in particular, to apparatus for regulating the bias potential on a development electrode in respect to a reference voltage sensed on an exposed xerographic plate prior to the plates being developed.

,In xerography, a plate having a photoconductive insulating layer thereon is imaged by first uniformly electrostatically charging the surface and then exposing the surface to a pattern of activating electromagnetic radiation, such as light. The radiation selectively dissipates the charge in the illuminated areas on the photoconductive surface while leaving behind a latent electrostatic image in the nonilluminated ares. The latent electrostatic image may then be developed to form a visible image by depositing finely divided electrostatic marking particles commonly called toner." on the surface of the photoconductive insulating layer.

' One of the most successful methods of developing a latent electrostatic image is by means of the two component development technique as disclosed by Wise, in US. Pat. No. 2,618,552. Two component development is based upon the phenomena of triboeleetrification. By rubbing together two triboelectrically dissimilar materials, an opposite electrostatic charge is induced in each of the materials. In xerography, finely divided toner particles are mixed with relatively coarser carrierbeads so that the toner particles are charged to a polarity opposite that of the latent electrostatic image. The two component material is then brought into contact with the exposed plate where the carrier beads give up their toner particles to the more highly charged imaged areas retained on the plate surface thus making the images visible.

The use of two component developer material has heretofore been limited to use in a cascade" development system as disclosed by Walkup, in u.S. Pat. No. 2,638,416. In conventional cascade development, the developer material is allowed to flow over an image retaining plate surface where the image is first developed in the manner disclosed by Wise. However, after image development, the toner deplete carrier beads still retaining a toner attracting charge, are allowed to clean or scavenge weakly held toner particles from the background or 'nonirnaged areas on the plate.

' A new development technique has recently been devised for controlling the activity of a two component developer flow as it moves in contact with an image retaining member. In this process, the developer material is caused to flow between the image bearing plate surface and a series of control electrodes. By regulating the bias potential on the individual electrodes, the degree of development or cleaning within predetemiined regions of the development zone is controlled. Because xerographic development is primarily dependent on the potential difference between background an image voltage, rather than on absolute values, the biasing potential placed on a control electrode, depending on its function, is generally maintained at some level above or below one of these voltages. However, it has been found that the electrical characteristics of most xerographic plate materials will change as the plate temperature changes or with extended plate usage thereby making it extremely difficult to maintain a uniform quality of development in this type of system.

It is therefore an object of this invention to improve/apparatus for developing a latent electrostatic image.

Another object of this invention is to develop uniform quality images continually in an electroded xerographic development system.

Yet another object of this invention is to improve electroded two component development for use in automatic xerographic reproducing machines.

A still further object of this invention is to sense changes in the electrical characteristics of a'xerographic plate and to vary the bias on a development electrode in response to the changes in order to maintain a uniform quality of development in an automatic xerographic reproducing machine.

These and other objects of the present invention are attained by a development electrode positioned in close spaced relation to a latent image retaining plate within a development zone, means to periodically sense a reference voltage on the plate, and means to regulate the bias potential on the electrode in response to the reference voltage sensed, to maintain the electrode voltage at a predetermined level in relation to the reference voltage.

For a better understanding of the present invention, as well as other objects and further features thereof, reference is had to the following detailed description of the invention to be read in connection with the accompanying drawings, wherein:

FIG. 1 illustrates schematically a xerographic reproducing apparatus adapted for high-speed automatic operation which incorporates the apparatus of the present invention;

FIG. 2 is a front elevation in partial section of the development system illustrated in FIG. 1 showing the development electrodes and the control apparatus of the present invention;

FIG. 3 is a partial side elevation of the sensing probe and shutter mechanism taken along lines 3-3 in FIG. 2;

FIG. 4 is an electrical diagram of the circuitry for periodically sampling the background voltage on the plate and for holding the sample between periods;

FIG. 5 is an electrical diagram of the circuitry for controlling the sample and hold circuitry shown in FIG. 4.

As shown in FIGS. 1 and 2, the automatic xerographic reproducing apparatus comprises a xerographic plate including a photoconductive layer of a light sensitive material placed on a conductive layer of a light sensitive material placed on a conductive backing and formed in the shape of a drum which is generally designated 10. The drum is joumaled for rotation in the machine frame (not shown) upon a horizontal support shaft 11. The xerographic drum is rotated in the direction indicated in FIG. 1 to cause the photoconductive surface to pass sequentially through a plurality of xerographic processing stations.

For the purpose of the present disclosure, the several xerographic processing stations in the path of movement of the drum surface may be described functionally as follows:

A charging station A, in which a uniform electrostatic charge is deposited on the moving photoconductive surface;

An exposure station B, wherein the light image or radiation pattern of an original document to be reproduced is projected on the drum surface to dissipate the charge found thereon in the light exposed areas so as to form a latent electrostatic image which is retained thereon;

A developing station C, at which a two component xerographic developing material having toner particles possessing electrostatic charge opposite to the image charge found on the drum surface are cascaded over the upwardly moving drum surface whereby the charged toner particles adhere to the electrostatic latent image areas making the images visible in the configuration of the original to be reproduced;

a transfer station D, in which the xerographic powder image is electrostatically transferred from the drum surface to a final support material; and

a drum cleaning and toner collecting station E, where the drum surface is first treated with a corona discharge to neutralize any residual charge found thereon and then cleaned with a flexible cleaning blade to remove residual toner from the drum surface, a more for collecting electrodes storing the removed residual toner and an incandescent panel to affect substantially complete discharge of any residual electrostatic image remaining thereon.

The charging station is preferably located at the bottom of the drum in the position indicated by reference A shown in FIG. 1. The charging arrangement consists of a corona charging device 13 which includes a corona discharge array of one or more corona discharge electrodes that extend transversely across the drum surface and are energized by a high potential source. The corona discharge device is substantially enclosed within a shielding member and is adapted to generate a positive charge confined within this specific area.

Next subsequent thereto in the path of drum rotation is an exposure station B wherein a flowing light image of a stationary original is placed on the drum surface. Basically, the exposure station comprises an optical scanning and projecting assembly and a stationary transparent copyboard 14 adapted to support the original to be reproduced. A moving light source 15 is mounted below the copyboard and is arranged to move in timed relation with a lens element 18 to scan the original supported upon the copyboard thus creating a flowing light image of the original. The light image is projected by the lens through a folded optical system, including an object mirror 19 and an image mirror 20, arranged to focus the light image on the bottom of the drum.

Positioned adjacent to theexposure station is a developing station C in which is positioned a developer housing 22 having a reservoir area therein capable of supporting a quantity of two component developer material including negatively charged toner particles. A bucket-type conveyor 23 transports developer material from the lower reservoir area to the upper part of the developer housing where it is deposited in entrance chute 21. Any suitable drive means can be used to rotate the bucket conveyor in the direction indicated. As will be explained in greater detail below, the developer material moves downwardly in contact with the upwardly moving photoconductive drum surface through a completely electroded development zone wherein the latent electrostatic image on the drum surface is developed. The unused developer material pases from the development zone and is directed back into the reservoir area by means of pickoff baffle 28. A toner container and dispensing apparatus 26 is affixed to the developer housing and is adapted to add fresh toner material into the reservoir area in proportion to the amount of toner deposited on the drum surface.

An image transfer station D is positioned adjacent to the developing station. Individual sheets of final support material are fed seriatim into the sheet registering and forwarding apparatus, generally referenced by numeral 27, from either of two supply trays 36 and 27. The individual sheets are properly registered and then forwarded into moving contact with the rotating drum surface and the developed electrostatic image transferred from the drum to the final support material by means of a transfer corotron 25. In operation, the electrostatic field created hy the corona discharge device electrostatically tacks or bonds the transfer material to the drum surface wherein the transfer material is caused to move in synchronous relation with the rotating drum surface.

A mechanical stripper finger 28 is pivotally mounted in close proximity to the drum surface immediately downstream from the transfer station. The stripper finger is arranged to move between the copy sheet and the drum surface breaking the electrostatic bond holding the sheet to the drum and to direct the support material into moving contact with the bottom surface of a stationary vacuum transport 29.

A combination of heat and pressure energy is employed in the present apparatus to fix the xerographic image to the final support material. The image bearing support material is guided into the fusing assembly 33 as it is moved along the bottom surface of transport 29. Fuser assembly 30 comprises an upper fuser roll 34 and a lower fuser roll 35 arranged to coact to deliver a pressure driving force to a sheet introduced therebetween. A radiant heat source 38 is positioned transverse to the lower fuser roll and applies heat energy to the surface of the roll. The roll, which is specially coated, stores the heat energy on its surface. As the rolls are rotated in the direction indicated, both heat energy and pressure energy are delivered by the roll into the imaged areas thereby fixing the image to the final support material. After leaving the fuser assembly, the now fixed copies are transported through a circular paper path, as illustrated in FIG. I, into a catch tray 39 where the copy can be conveniently collected by the machine operator. 7

Referring more specifically to FIGS. 2 through 4, there is illustrated a xerographic development apparatus in which a latent electrostatic image retained on an upwardly moving photoconductive drum surface is developed by means of a two component developer material moving downwardly in contact with the drum surface. Although this flow relationship between the plate surface and the developer material is reversed from that found in most conventional two component developer systems, rapid and efficient image development is accomplished in the present apparatus because of the unique control characteristics of the developing system herein disclosed.

As previously noted, the two component developer material is first transported from the reservoir or storage area in developer housing 22 and deposited in a hopperlike input chute 21 by means of a bucket conveyor system 23. A quantity of developer material is stored within the input chute and flows downwardly through a constrained opening 41 into the introductory region of development zone 40. As illustrated in FIG. 2, the front wall of the development zone is formed by the movable drum surface 10 while the rear wall is formed by a series of downwardly extended electrodes running transversely across the photoconductive coating on the drum surface. The electrodes are supported in spaced parallel relation to the drum surface by means of an insulating support frame 43 secured to the walls of the developer housing by any suitable means. The individual electrodes are separated from each other by dielectric blocks 42 so that the rear wall of the development zone presents a substantially continuous surface to the developer material introduced therein. Although not shown, end seals are provided between the electrodes and the drum surface to substantially enclose the development zone thus providing a conduit through which the developer material gravity flows. The development zone extends from the introductory opening 41 opposite to the upper drum surface to a point well below the horizontal centerline of the drum.

Basically the control electrodes are biased so that the developer material performs a cleaning function in the upper development zone while a preponderance of image development takes place in the lower inverted development zoneiegion thereof. By varying the charge potential and magnitude on the various electrodes, the concentration and positioning of toner in the flow stream can be controlled to regulate the degree of development and cleaning obtained in each of the electroded regions.

The first electroded region through which a latent electrostatic image is transported is the region influenced by a lowpotential electrode 45 physically located in the bottom of the development zone 40. The term low potential, as herein used, refers to a potential which is lower that the background potential on the xerographic plate surface. This term is broad enough to include a grounded electrode or even a floating electrode. Because of the unique control features of the present developing apparatus, carrier beads which are properly toned for optimum development are flowing through this lower development zone. In this preferred embodiment, the low-potential electrode is placed at a ground potential so that an extremely strong force field is established tending to force the negatively charged toner particles toward the plate side of the development wne. At the same time, the electrode acts as a conventional development electrode to enchance the latent electrostatic force fields, particularly the force field associated with solid imaged areas, so that extremely rapid and efficient image development is produced in this region.

The leading edge of the low-potential electrode, that is, the edge that first presents itself to the developer flow, is chamfered to direct the developer flow upwardly into contact with the drum surface. In this manner, toner particles are both physically dislodged from the carrier beads and transported into contact with the plate surface. The airborne toner particles, because they are in a free state, are readily attracted into the image areas so that extremely rapid development takes place in this region. Overdevelopment of the xerographic plate, in fact, may result. However, as will be explained below, an overdeveloped condition in this region can be tolerated by the present development system.

The next electrode positioned in the direction of drum rotation is the main development electrode 46. The main development electrode is biased at a potential somewhere between the image potential and the background potential found on the plate surface and preferably at some predetennined level above the background voltage. When an imaged area on the drumsurface is transported through the main development electrode region, the force field associated with the imaged area, being of a higher magnitude than the electrode force field, predominates. The toner in the flow stream adjacent to the imaged surface is thus attracted into the imaged areas. However, when a nonimaged or background area is moved through the main developing region, the electrode force field dominates and the toner particles are pulled away from the plate surface towards the backside of the development zone. The developer material moving in contact with the nonimaged drum surface therefore tends to mechanically scrub the background areas to dislodge randomly dispersed, weakly held, toner particles from the plate. This dislodged toner, coming under the influence of the stronger electrode force field, is similarly attracted towards the electroded side of the system. As can be seen, the main development electrode, in effect, acts as a self-resulting device to either complete image development or to cleanup background areas in this region.

The now xerographically developed photoconductive surface next moves into the last development region in which an extremely strong toner attracting force field is produced by a cleanup electrode 47. A biasing source 44 is electrically connected to the cleanup electrode and electrically biases electrode at apotential greater than the image potential on the plate surface, preferably 300 volts above the image potential. The-bias potential is sufficiently high enough to attract an extremely heavy concentration of toner in the flow stream to the backside of the development zone. The carrier beads moving in contact with the plate surface become toner depleted and therefore are capable of both mechanically scrubbing and electrostatically scavenging unwanted background development from the plate surface. Here again, the strong electrode force field attracts random toner particles from the vicinity of the plate surface such that a clear well-defined developed xerographic image leaves the development zone.

As shown in FIG. 2, cleanup electrode 47 is turned at a slight radium at the developer entrance 41 and extends outwardly and upwardly from the development zone to form the bottom wall of the input chute 21. The opposite wall of the input chute is formed by an electrically isolated baffle 48 secured to the developer housing wall by suitable means. The lower end of the baffle has a lip formed thereon complementary to the turning radius of the cleanup electrode so that a unifonn opening 41 is provided through which the developer material enters the development zone in a relatively undisturbed flow. Baffle 48 is placed at a ground or toner repelling potential which, when combined with the toner attracting force field of electrode 47, forces a preponderance of the toner particles in the flow to the backside of the system. Because of the input chutes physical configuration and the strong electrostatic force field associated therewith, the formation of toner powder clouds in and about the introductory region to the xerographic development zone is minimized thus preventing unwanted background development from occuring. A strong toner concentration is thus established on the backside of the flow stream prior to the developer material entering the development zone so that relatively toner deplete beads initially contact the drum surface as it leaves the development zone.

The electrostatic properties of many known photoconductuve plates tend to change slightly with changes in temperature or with extended plate usage. This change or drifting in the electrical plate parameters has little or no effect on the control features of the low-potential electrode or the cleanup electrode. However, this is not the case in regard to the main development electrode. As noted, the main development electrode is held at some predetermined voltage between plate image voltage and plate background voltage and preferably at some fixed voltage above the plate background voltage. Here the difference between the reference voltage and the desired electrode voltage is small and any electrical drifting in the plate voltage will normally be reflected in a change in the quality of development produced.

Circuitry is herein provided to regulate the bias potential on the main development electrode in order to compensate for changes in the plate voltage so that images of uniform quality are produced by the present development apparatus'Physically, the main development electrode control system comprises: a sensing probe adapted to periodically sample the level of background voltage on the rotating drum surface, a signal generating device adapted to convert the sampled voltage into a constant control signal; and an adjustable power supply responsive to the control signal wherein the development electrode is maintained at a predetermined voltage level in regard to the sampled plate voltage.

A sensing probe support housing 49 is secured in the machine frame and is positioned between the xerographic ex posure station and the developing station. The sensing probe 50 (FIG. 3) is seated within the support housing in juxtaposition to one end of the drum surface and is arranged to sense a narrow sample strip on the photoconductive surface near the edge of the drum.

The sample strip is arranged to pass through the charging and exposing stations and, as a result, is placed at the plate background potential. The strip, however, is sufficiently offset toone side of the drum surface so that its presence does not interfere with the normal machine operations.

A solenoid actuated shutter 53 is slideably mounted within the guides provided in the upper portion of the support housing. The shutter is operatively connected to a solenoid SOL-l by means of a crank arm 54. The crank arm is rotatably mounted upon a pivot pin 55 and the pin secured in the body of the housing. The lower end of the crank arm is pivotally affixed to the solenoid actuator arm 56 while the opposite end of the arm is similarly connected to a downwardly turned dependent flange 58 formed in the lower part of shutter 53. A pin 57, passing through the upper part of the crank arm, rides in a vertically aligned slotted hole (not shown) formed in flange 58 which permits the shutter to move in a horizontal direction as the crank arm is rotated. In operation, the solenoid is energized once curing each copying cycle. As illustrated in FIG. 3, energization of the solenoid pulls actuator arm 56 upwardly causing the crank arm to rotate in a counterclockwise direction. whereby, the shutter is moved back exposing the sampling probe to the drum surface.

The machine logic system, generally referenced in FIG. 2, is arranged to generate a midscan trigger signal during each xerographic copying cycle. in practice, the signal is generated as the scanning lens 18 physically passes the midpoint of its programmed path of travel and the trigger signal is passed to the sample and hold circuitry 51. A voltage indicative of the plate background voltage is sensed by the probe and this used to generate a continuous output control signal which is applied to a variable power supply 52. The power supply is operatively connected to the main development electrode 46 and regulates the electrode potential at a predetermined level above the background voltage on the plate.

The midscan trigger signal generated by the machine logic system has a pulse duration of approximately 0.5 seconds and recurrence of about 1.5 seconds between automatic copying cycles. initially, the trigger signal is sent to the sample and hold control circuitry where it is applied to input terminal 94 (FIG. 5). The signal taken passes to the base of transistor 65 causing the transistor to fire. The transistor is held conductive for the duration of the trigger pulse signal and allows current to energize shutter relay coil 66. With the shutter coil energized, the actuator arm of solenoid SOG-l (FIG. 3) is pulled upwardly causing the shutter to move back thereby exposing probe 50 to the drum surface. At this time, the probe sees the background voltage on the sample strip and sends a voltage signal indicative of this voltage to a unity gain electrometer amplifier 8I. Any suitable electrometer amplifier commercially available through a number of manufacturing sources having a very high impedance can be utilized in conjunction with the circuitry of the present invention. The output of the electrometer amplifier is passed through two more amplifier stages 83 and 84 and then applied to a hold circuit comprised of a high-impedance unity gain, amplifier 86 and a capacitor 87. The signal, however, is initially prevented from passing to the hold circuitry by a normally open contact 85.

In practice, an output signal between 1.5 and 6.5 volts is generated through the probe and amplifier circuit which is indicative of a proportional background voltage of between and 500 volts.

As shown in FIG. 4, probe 50 basically comprises a sensing element 92 which is completely surrounded by an insulator 93. The insulator is preferably fabricated of a material which is electrically insensitive to changes in humidity and functions to maintain a high probe-to-ground resistance. A conductive shield 91 is also placed around the insulator material and the output from the unity gain electrometer amplifier fed back to this shield. By maintaining the shield at the same potential as the electrometer amplifier output in this manner a bootstrap effect, is produced causing in probe shunt capacitance and reduces current leakage from probe surface 92 to the surrounding electrical ground,

The midscan trigger pulse is also applied to the base of transistor 60 which is in the control circuitry for opening and closing contact 85. The trigger pulse causes the transistor to conduct and an output signal from the emitter side of the transistor applied to a monostable multivibrator 61. The monostable multivibrator holds or delays the input signal for approximately 0.16 seconds during which time the shutter is moved to a fully opened position. The signal is then passed to a second monostable multivibrator 62 which, upon receiving the delayed signal, produces an output pulse signal having a pulse duration of about 0.22 seconds. The output pulse from multivibrator 62 is applied to the base of transistor 64 causing the transistor to fire. Current is thus allowed to energize coil 63. With relay coil 63 energized, contact 85, in the hold circuitry, is pulled to a closed position and the sample voltage applied across high impedance unity gain amplifier 86 (FIG. 4).

Closing contact 85 causes two discrete occurrences to take place. First, as noted above, the sensed sample voltage is impressed across the high-impedance amplifier 86 andsecondly capacitor 87, in the hold circuitry, is also charged to the sample voltage. Termination of the 0.22 second pulse signal closes the sample window by cutting off transistor 64. The circuit to relay coil 63 is now broken and contact 85 allowed to return to an open position. The initial sampled voltage, however, which is sorted on capacitor 87, continues to be impressed across amplifier 86. Because of the high impedance of the amplifier, a relatively constant output is maintained during the hold period until the subsequent reclosing of contacts 85 provide a new sample voltage. This output voltage is applied to the power supply high-voltage operation amplifier 73 (FIG. 4) holding the power supply relatively constant during the short period the sample and hold circuit is waiting for the next sample signal.

If the voltage level of the next sample signal differs from that of the first sample, capacitor 87 is allowed to recharge to the new potential through contact 85 through the circuitry of amplifier 84. The new samplevoltage is impressed across the high-impedance hold amplifier 86 and the capacitor is now charged to the new voltage. At the end of the sample period, contact 85 is again opened and the hold circuit again waits for the next sample. As can be seen, the above arrangement makes it possible for the present apparatus to sense both increases and decreases in drum potential while at the same time continually generating a control signal for regulating the main developing electrode potential.

Further circuitry is also herein provided to reduce the electrical drift in the electrometer amplifier between sampling periods. As shown in FIG. 5, the trigger pulse is also applied to the base of transistor 78 causing the transistor to conduct. A inverted output signal is sent from the collector side of the transistor to the first of a pair of series coupled monostable multivibrators 74 and 75. Upon termination of the trigger signal, monostable multivibrator 74 is activated to provide a pulse of duration of about 0.24 seconds. During this delay period, the shutter solenoid coil is deenergized and the shutter is allowed to fully close. The delayed signal is then passed to the'second monostable multivibrator 75 which produces an output signal having a pulse duration of approximately 0.24 seconds.

The output signal from monostable multivibrator 75 is applied to the base of a second transistor 79. Transistor 79 transfers an inverted output signal to the gating network made up of diodes 72 and 73. During normal machine cycling, the inverted signal is conducted through diode 73 and transistor to provide a grounding signal for the electrometeramplifier 81. This signal is applied to a gating arrangement (not shown) in the electrometer amplifier circuit which grounds the input circuit for the duration of the 0.24 second delay during which time probe surface 92 senses the ground potential of the closed shutter. The very high-impedance input circuit of amplifier 81 is thus provided with a zero potential reference for r the 0.24 second duration of this gate signal. In this manner, amplifier 81 is provided with an accurate zero potential reference once during each machine cycle (l.5 seconds). Thus reference technique eliminates errors due to the long term electrical drift characteristics of amplifier 81.

When a copy run is initiated, that is, when lens 18 scans an original on the platen, the trigger pulse is also impressed upon the base of transistors 68 and 69, which are caused to conduct. The voltage developed at the emitter of transistor 69 is applied to diode 72. Diodes 72 and 73 fonn a gate which will pass only the lower of the two input voltages to the base of transistor 80, and thus to the gage of amplifier 81. The reverse biased diode appears as a high impedance to the signal generated by monostable multivibrator 75 and the signal thus passes through the diode gating network to transistor 80. A capacitor 71, located in the base circuit of transistor 68 holds transistors 68 and 69 conductive during the short period between copying cycles thus holding diode 72 in a reversed biased condition during this period. In this condition, the 0.24 second gate pulse passes through diode 73 and transistor 80 to amplifier 81 as previously described. However, inactivation of the scanning system for any extending period of time causes capacitor 71 to discharge and diode 72 passes a zero volt signal from the emitter of transistor 69 to the base of transistor 80. Transistor 80 turns off at this time causing a grounding signal to be sent to the electrometer amplifier gate. As can be seem the electrometer amplifier is continuously grounded by the control circuit any time the machine is on except during the time copies are being produced and a continuous train of trigger pulses is arriving at point 94 (FIG. 5).

While this invention has been disclosed with reference to sampling the background voltage on the plate surface, it should be clear that it is not confined to the specific details as set forth. It should be quite clear that the main development electrode, or any of the control electrodes, can be controlled in regard to any desired reference voltage on the plate and this application is intended to cover such modification or changes as may come within the purposes of the present invention. Furthermore, for purpose of convenience, throughout this specification, reference has been made to positively charged carrier material and negatively charged toner particles. It is to be understood that this description of the nature of the charge, with respect to the electrodes, is not intended to limit this invention to this specific relationship. It would be possible to utilize carrier materials and toner materials having a different charge relationship in respect to the triboelectric properties whereby the carrier could be charged negatively and the toner positively thereby their requiring a similar change in the relationship of the various biasing electrodes.

What is claimed is:

1. In a xerographic developing apparatus of the type wherein a moving latent image retaining member is brought into contact with a flow of charged developer material within a development zone, the apparatus including an electrically isolated electrode supported in the development zone and being positioned in spaced relation to the image retaining member wherein the flow of developer material moves between said electrode and said member,

electrical means to bias said electrode,

a probe arranged to sense a reference voltage retained on said member prior to said member moving through the development zone,

means to periodically sample the reference voltage and convert the sampled voltage to an output signal indicative of said reference voltage,

shudder means positioned between said probe and said image retaining member having actuator means associated therewith to move said shudder means be between said probe and said image retaining member between each sampling period, and

means to regulate said biasing means in response to the output signal wherein said electrode is maintained at a predetermined voltage level in relation to the sampled reference voltage.

2. The apparatus of claim 1 wherein said shudder is placed at a ground potential.

3. The apparatus of claim 2 further including means to move the latent image retaining member in a direction opposite to the direction of developer flow.

4. The apparatus of claim 3 wherein the voltage sampled is the background voltage on the image retaining member.

5. In a xerographic supported apparatus of the type wherein a moving latent electrostatic retaining member is brought retaining contact with a .flow of charge developer material within a developing mne, the apparatus including an electrically isolated electrode supported in the development zone and beingpositioned in spaced relation to the image retaining member wherein the flow of developer material moves between the electrode and said member,

electrical means to bias said electrode,

a probe arranged to sense a reference voltage retained on said member prior to said member moving through the development zone,

means to periodically sample the reference voltage,

circuit means for analyzing the periodically sensed reference voltage and converting said periodic reference voltage to a continuous output signal indicative of the reference voltage, and

means to regulate said electrical bussing means in response to the continuous output signal to maintain said electrode continually at a constant predetermined voltage level in relation to the sampled reference voltage.

6. The apparatus of claim 5 further including a control means to ground said circuit mean between sample periods to reduce the electrical drift within said circuit.

7. Apparatus for controlling the development of a latent electrostatic image on an image retaining member, the development being produced by electroscopic developer material moving in contact with the image retaining member, said apparatus including an electrically isolated electrode supported in the development zone and being positioned in spaced relation to the image retaining member wherein developer material moves between said electrode and said image retaining member,

biasing means operatively associated with said electrode for maintaining said electrode at a potential level between the image charge potential and the nonimage charge potential found on the image retaining member,

sensing means to sample the charge potential found in the nonimaged areas of the image retaining member, and

circuit means connecting said sensing means to said biasing means for converting the sample signal to a continuous Output signal for maintaining said electrode at a predetermined reference voltage above the nonimage charge potential found on said image retaining member and below the image charge potential found on said image retaining member.

7% UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No. 3,611, 982 Dated October 12, 1971 Inventor(s) Samuel Coriale and Ned J. Seachman It is certified that error appears in the above-identified patent and that said Letters Patent are hereby corrected as shown below:

Title page, under [72] Inventors delete the residence of Ned J. Seachman as being "Penfield, N.J."

and substitute therefore -Penfie1d, N.Y.-.

Signed and sealed this 13th day of August 197 (SEAL) Attest:

MCCOY M. GIBSON, JR. Attesting Officer C. MARSHALL DANN Commissioner of Patents 

1. In a xerographic developing apparatus of the type wherein a moving latent image retaining member is brought into contact with a flow of charged developer material within a development zone, the apparatus including an electrically isolated electrode supported in the development zoNe and being positioned in spaced relation to the image retaining member wherein the flow of developer material moves between said electrode and said member, electrical means to bias said electrode, a probe arranged to sense a reference voltage retained on said member prior to said member moving through the development zone, means to periodically sample the reference voltage and convert the sampled voltage to an output signal indicative of said reference voltage, shudder means positioned between said probe and said image retaining member having actuator means associated therewith to move said shudder means be between said probe and said image retaining member between each sampling period, and means to regulate said biasing means in response to the output signal wherein said electrode is maintained at a predetermined voltage level in relation to the sampled reference voltage.
 2. The apparatus of claim 1 wherein said shudder is placed at a ground potential.
 3. The apparatus of claim 2 further including means to move the latent image retaining member in a direction opposite to the direction of developer flow.
 4. The apparatus of claim 3 wherein the voltage sampled is the background voltage on the image retaining member.
 5. In a xerographic supported apparatus of the type wherein a moving latent electrostatic retaining member is brought retaining contact with a flow of charge developer material within a developing zone, the apparatus including an electrically isolated electrode supported in the development zone and being positioned in spaced relation to the image retaining member wherein the flow of developer material moves between the electrode and said member, electrical means to bias said electrode, a probe arranged to sense a reference voltage retained on said member prior to said member moving through the development zone, means to periodically sample the reference voltage, circuit means for analyzing the periodically sensed reference voltage and converting said periodic reference voltage to a continuous output signal indicative of the reference voltage, and means to regulate said electrical bussing means in response to the continuous output signal to maintain said electrode continually at a constant predetermined voltage level in relation to the sampled reference voltage.
 6. The apparatus of claim 5 further including a control means to ground said circuit mean between sample periods to reduce the electrical drift within said circuit.
 7. Apparatus for controlling the development of a latent electrostatic image on an image retaining member, the development being produced by electroscopic developer material moving in contact with the image retaining member, said apparatus including an electrically isolated electrode supported in the development zone and being positioned in spaced relation to the image retaining member wherein developer material moves between said electrode and said image retaining member, biasing means operatively associated with said electrode for maintaining said electrode at a potential level between the image charge potential and the nonimage charge potential found on the image retaining member, sensing means to sample the charge potential found in the nonimaged areas of the image retaining member, and circuit means connecting said sensing means to said biasing means for converting the sample signal to a continuous output signal for maintaining said electrode at a predetermined reference voltage above the nonimage charge potential found on said image retaining member and below the image charge potential found on said image retaining member. 